212 ANALYTICAL MECHANICS the potentials due to a negative charge and a negative pole are negative, while the potentials due to a positive charge and a positive pole are positive. 179. Potential Due to Any Distribution of Mass. When the field of force or the potential field is due to a number of particles (material, electrical, or magnetic), the potential at a point equals the algebraic sum of the potentials due to the various particles. Thus if mi, m^ ra3, etc., be the masses of the particles and n, r2, r3, etc., their distances from the point considered, then the potential at the point is / mi = ly Tl (VII) When the field is due to a continuous distribution of mass the last equation may be put in the form of an integral. Thus 7= -7 - (VII7) JQ T 180. Intensity of the Field. The intensity at any point of a potential field, or a field of force, is defined as the force experienced by a unit mass when placed at that point. Let H denote the intensity at a point. Then, if F is the force experienced by a mass m! when placed at that point, we have, by definition, -> (VIII) m } and #= m --(-} m'\ds] ds \m'